This invention relates to a method of and apparatus for dispensing fluids and, more particularly, to a method of and apparatus for producing droplets on demand.
Methods of forming and dispensing of droplets of liquids have been used in many applications including ink-jet printing and the precision dispensing of chemicals and pharmaceuticals. More recently, the basic technology has been utilized to provide accurate placement of solder on printed circuit boards (PCBs) and electronic components. For example, U.S. Pat. No. 5,415,679 to Wallace, describes an ejection device which includes an electrostrictive transducer which, when activated by a particular waveform, can be used to produce and apply droplets of solder to a substrate. The particular waveform described by Wallace is shown in FIG. 1. This waveform is characterized by a first rise time RT1, followed by a first dwell time DT1, followed by a fall time FT, followed by a second dwell time DT2 and a final rise time RT2 back to the nominal position.
The process by which a droplet is generated when the prior art waveform drives the transducer is well known. An expansion wave is generated during the first rise time RT1 and this wave splits into two waves which travel in opposite directions back and forth along the axis of the ejector. Each time a wave hits a tube end, a pressure inversion occurs (i.e. an expansion becomes a compression, and vise versa). A stable droplet is generated only if the compression wave generated during the fall time FT is synchronized with the passage of a compression wave traveling towards the orifice. This implies that the first dwell time DT1 of the waveform shown in FIG. 1 is critical to the formation of a droplet. Similarly, the expansion wave generated during the final rise time RT2 has to be synchronized by tuning the second dwell time DT2. In principle, the purpose for this expansion is to minimize or even cancel any residual pressure waves in the tube after a droplet is generated.
Developing practical droplet forming systems using the waveform shown in FIG. 1 has been difficult because determining the right dwell times DT1 and DT2 is difficult and typically requires continuous tuning which involves significant intervention of a trained operator. In addition, the jetting parameters necessary to achieve a stable system, Pressure, DT1, DT2, and Voltage, can vary from device to device making automatic tuning difficult.
Accordingly, it is an object of this invention to provide an improved method of and apparatus for producing droplets.
It is another object of this invention to provide an improved method of and apparatus for producing droplets which utilize a simplified waveform.